Intravascular ultrasound (IVUS) imaging is widely used in interventional cardiology as a diagnostic tool for assessing a diseased vessel, such as an artery, within the human body to determine the need for treatment, to guide the intervention, and/or to assess its effectiveness. IVUS imaging uses ultrasound echoes to form a cross-sectional image of the vessel of interest. Typically, the ultrasound transducer on an IVUS catheter both emits ultrasound pulses and receives the reflected ultrasound echoes. The ultrasound waves pass easily through most tissues and blood, but they are partially reflected by discontinuities arising from tissue structures (such as the various layers of the vessel wall), red blood cells, and other features of interest. The IVUS imaging system, which is connected to the IVUS catheter by way of a patient interface module, processes the received ultrasound echoes to produce a cross-sectional image of the vessel where the catheter is located.
There are two types of IVUS catheters in common use today: solid-state and rotational, with each having advantages and disadvantages. Solid-state IVUS catheters use an array of ultrasound transducers (typically 64) distributed around the circumference of the catheter and connected to an electronic multiplexer circuit. The multiplexer circuit selects array elements for transmitting an ultrasound pulse and receiving the echo signal. By stepping through a sequence of transmit-receive pairs, the solid-state IVUS system can synthesize the effect of a mechanically scanned transducer element, but without moving parts. Since there is no rotating mechanical element, the transducer array can be placed in direct contact with the blood and vessel tissue with minimal risk of vessel trauma and the solid-state scanner can be wired directly to the imaging system with a simple electrical cable and a standard detachable electrical connector.
In the typical rotational IVUS catheter, a single ultrasound transducer element fabricated from a piezoelectric ceramic material is located at the tip of a flexible driveshaft that spins inside a plastic sheath inserted into the vessel of interest. The transducer element is oriented such that the ultrasound beam propagates generally perpendicular to the axis of the catheter. The fluid-filled sheath protects the vessel tissue from the spinning transducer and driveshaft while permitting ultrasound signals to freely propagate from the transducer into the tissue and back. As the driveshaft rotates (typically at 30 revolutions per second), the transducer is periodically excited with a high voltage pulse to emit a short burst of ultrasound. The same transducer then listens for the returning echoes reflected from various tissue structures, and the IVUS imaging system assembles a two dimensional display of the vessel cross-section from a sequence of several hundred of these ultrasound pulse/echo acquisition sequences occurring during a single revolution of the transducer.
While the solid-state IVUS catheter is simple to use, thanks to its lack of moving parts, it cannot match the image quality available from a rotational IVUS catheter. It is difficult to operate a solid-state IVUS catheter at the same high frequency as a rotational IVUS device, and the lower operating frequency of solid-state IVUS catheters translates into poorer resolution compared to that of a higher frequency rotational IVUS catheter. There are also artifacts such as sidelobes, grating lobes, and poor elevation focus (perpendicular to the imaging plane) that arise from the array-based imaging that are greatly reduced or completely absent with a rotational IVUS device. Despite the image quality advantages of the rotational IVUS catheter, each of these devices has found a niche in the interventional cardiology market, with solid-state IVUS preferred in circumstances where ease-of-use is paramount and the reduced image quality is acceptable for the particular diagnostic needs, while rotational IVUS is preferred where image quality is paramount and the more time-consuming catheter preparation is justified.
In the rotational IVUS catheter, the ultrasound transducer is typically a piezoelectric ceramic element with low electrical impedance capable of directly driving an electrical cable connecting the transducer to the imaging system hardware. In this case, a single pair of electrical leads (or coaxial cable) is used to carry the transmit pulse from the system to the transducer and to carry the received echo signals from the transducer back to the imaging system by way of a patient interface module, where they are assembled into an image. An important complication in this electrical interface is the transportation of electrical signals across a rotating mechanical junction. Since the catheter driveshaft and transducer are spinning (in order to scan a cross-section of the artery) and the imaging system hardware is stationary, there must be an electromechanical interface where the electrical signals traverse the rotating junction. In rotational IVUS imaging systems, this problem can be solved by a variety of different approaches, including the use of a rotary transformer, slip rings, rotary capacitors, etc.
While existing IVUS catheters deliver useful diagnostic information, there is a need for enhanced image quality to provide more valuable insight into the vessel condition. For further improvement in image quality in rotational IVUS imaging, it is desirable to use a transducer with broader bandwidth and to incorporate focusing into the transducer. The piezoelectric micromachined ultrasound transducer (PMUT) fabricated using a polymer piezoelectric material, such as disclosed in U.S. Pat. No. 6,641,540, offers greater than 100% bandwidth for optimum resolution in the radial direction, and a spherically-focused aperture for optimum azimuthal and elevation resolution. While this polymer PMUT technology offers many advantages, the electrical impedance of the transducer is too high to efficiently drive the electrical cable connecting the transducer to the IVUS imaging system by way of the PIM. Furthermore, the transmit efficiency of polymer piezoelectric material is much lower compared to that of the traditional lead-zirconate-titanate (PZT) ceramic piezoelectric, therefore the signal-to-noise ratio will be compromised unless the deficiency in acoustic output can be compensated by improved transmit electronics or other signal processing advances.
Accordingly, there remains a need for improved devices, systems, and methods for providing a compact and efficient circuit architecture and electrical interface to a polymer piezoelectric micro-machined ultrasonic transducer used in an intravascular ultrasound system.